In ophthalmology it has been established, in case of defective vision, to form the cornea of the human eye with its approximate thickness of 500 μm through ablation of tissue in order to correct myopia, hypermetropia, and astigmatism. This is called refractive surgery. In addition, the transplant of pieces of the cornea from a donor eye into a recipient's eye has been established in order to replace diseased or damaged corneal tissue and therefore retain or restore vision. Both methods are collectively termed keratoplasty and can be executed by means of lasers. In the so-called lamellar keratoplasty, a single disc from a donor cornea is transplanted in or onto a recipient's cornea.
Anatomically, the human cornea consists of five different tissue layers. From anterior to posterior they are: Epithelium, Bowman's membrane, stroma, Descemet's membrane, and endothelium. Thereby, the stroma takes up the largest volume.
During laser-supported intrastromal keratomileusis (LASIK), a flap with an approximate thickness of typically 80 μm or more is detached from the cornea and folded up. For example, it is known from US 2006/0155265 A1 (Intralase Corp.) to cut the flap by means of a femtosecond laser system (Femto-LASIK). Such devices are also called laser microkeratomes. Thereby, a photodisruption is produced in the focus, which leads to a minimal formation of bubbles in the stromal tissue. If focal spot is set next to focal spot by means of a scanner system, incisions (perforations) can be made in the cornea.
The ablation of the stromal tissue, necessary for a refractive correction, is subsequently executed conservatively by means of an excimer laser. After treatment, the flap is folded back. Disadvantageously, said method requires two laser systems.
In WO 2008/064771 A1 (Carl Zeiss Meditec AG), a femtosecond laser system is described which can also prepare the flap but is additionally capable of separating the ablation of stromal tissue, necessary for a refractive correction, through multiple incisions for the preparation of a lenticle. This can be called femtosecond lenticle extraction. Subsequently, the lenticle can be removed after opening the flap, e.g., with a pair of pincers. Then the flap is folded back again. For said method, only one laser system is required, the use of an excimer laser can be forgone. Such a laser system also allows for the execution of incisions during the transplant of corneal tissue. Thereby, the femtosecond laser system can be used for incisions on the donor eye as well as for incisions in the recipient's eye.
During keratoplasty (refractive correction, transplant), the shape of the cornea of the human eye is problematic. For example, the refractive power of the cornea depends on its shape. The anterior corneal surface is in a first approximation a toric surface and is generally described through two radii of a certain axial position perpendicular to one another. The thickness of the cornea also increases towards its periphery. In addition, further irregularities of the corneal thickness can occur with certain pathologies.
According to prior art, for the planning of a laser-supported keratoplasty, the shape of the cornea is specially measured (pachymetry), e.g., contactless by means of a Scheimpflug camera or an optical coherence tomography system (OCT) for the anterior eye segment. The contacting measurement by means of ultrasound is also known. However, during the actual surgery, the cornea is held on the femtosecond laser through the application of a contact glass and suctioning of the eye, whereby, as a rule, the shape of the cornea is altered (applanation).
As a result, the parameters of the shape of the cornea, obtained outside of the actual surgery, are now, among others, only meaningful within limits. Therefore, the accuracy of the laser treatment, as far as it is based on the measured shape, is limited. As a result, the transplant can deviate from its planned shape and not be implanted optimally in the recipient's eye. This can lead to complications, e.g., the detachment of the transplant or glaucoma due to a shift of the transplant.
Regardless of the shape of the cornea, there is also the problem that in some cases optically opaque bubbles (opaque bubble layer—OBL) may appear during the perforation of corneal tissue by means of femtosecond laser radiation with lamellar incisions. This refers to an area in the immediate surroundings of the actual laser incision, whereby the forming of the laser-induced micro-gas bubbles not only occurs in the plane of the incision. Depending on position and characteristic of an OBL field, the perforation of the tissue in this area is not ideal. In comparison to the surrounding area, the result is a more difficult manual detachment of the tissue components to be separated, which leads to tissue stress and/or to a prolonged duration of surgery. Therefore, prior art attempts to avoid the formation of OBL. For example, this is accomplished through optimization of treatment parameters, such as pulse energy and spot distance and/or track distance. The modification of the incision geometry towards deeper incisions, e.g., greater flap thickness or the creation of a low-lying gas pocket, e.g., at a depth of 250 μm, can contribute to a decrease of the frequency of OBL. However, experience has shown that even under said conditions, OBL occurs in some eyes. Furthermore, modifications of the incision geometry and gas pockets are disadvantageous with regard to the preservation of as much residual stromal thickness as possible.